Lesson 3.2: Lewis Structures

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*the steps in drawing a Lewis structure:* if the general formula for the compound is "AXn," the central atom, _____, will be the one with the lower _____ (and usually the lower _____)

"A", group number, electronegativity

*how many total valence electrons are present in the following molecules?* NF₃

26

*how many total valence electrons are present in the following molecules?* SF₆

48

*how many total valence electrons are present in the following molecules?* H₂Se

8

give the formal charge of each atom in an HF molecule

H = 1 - (0 + 1) = 0 F = 7 - (6 + 1) = 0

dihydrogen monosulfide gas smells like rotten eggs with the formula, H₂S. explain why the Lewis structure skeleton cannot be S-H-H

H has only one valence electron that it can share in a single covalent bond do it can only form one bond

N₂O has three resonance forms whose structures are given below. use formal charges to identify which is the dominant form, A, B, or C: N//N//O N///N/O N/N///O

N///N/O

carbon dioxide has two possible structures that obey the octet rule. draw the Lewis structures and determine which is the most likely by calculating formal charges.

O//C//O

the greater _____ between the atoms in a multiple bond also brings the atoms closer together, so double and triple bonds are _____ than single bonds

attraction, shorter

double and triple bonds involve more _____ the single bonds as so are _____

attractive forces, stronger

for nonmetals, Lewis structures representing the anions include _____ pairs of dots surrounding the element's symbol enclosed in _____ with the negative charge written as a superscript to the right and outside the _____

four, brackets, brackets

molecules that have Lewis structures with unpaired electrons

free radicals

unpaired electrons, VERY reactive

free radicals

representations of atoms of elements based on their number of valence electrons

electron dot diagram/Lewis structure

_____ get brackets and charge

polyatomic ions

*electron dot diagram* NCl₃

-the molecule NCl₃ has three Cl atoms with 7 valence electrons each -the N atom has 5 electrons for a total of 26 valence electrons

*electrons remaining that need to be arranged* NCl₃

in NCl₃ 26 - 6 = 20 valence electrons remaining that need to be arranged in the structure

*the central atom* NCl₃

in NCl₃, N is in Group 15 while Cl is in Group 17, so N is the central atom

what makes free radical reactive?

its single electron

small or zero formal charges on individual atoms are _____ than larger ones

better

the amount of energy required to break a mole of these bonds is called _____ and is measured in _____

bond energy, kJ/mol

sometimes fewer electros are available than needed to complete the octets of the atoms in a molecule. the most important of these is _____ which forms compounds with only _____ around B, rather than _____

boron, 6, 8 ex: BF₃ lacks an octe for B

an expanded octet is only possible for an atom when _____ electrons are available, so they are only possible for elements in the _____ period or _____ periods

d orbital, third, higher

expanded octet only for elements with _____

d sublevels

electrons that are not associated with any one pair of bonded atoms, but are spread out equally between the three pairs of atoms

delocalized

_____ are depicted by showing the number of possible structures and connecting each structure with a _____. the diagrams are called ____.

delocalized electrons, double-headed arrow; resonance structures

two free radicals that combine

dimers

free radicals may form _____

dimers (N₂O₄)

when a molecule or ion contains _____ next to _____, there are often _____

double bonds, single bonds, several possible structures that we can draw that are all correct

two or three pairs of electrons may also be shared between certain atoms to form _____

double or triple bonds

for nonmetals, atoms need to gain enough electrons to have _____ (_____) when forming an anion OR the number of electrons the atoms needs to _____

eight dots (stable octet), share when forming covalent bonds

*the steps in drawing a Lewis structure:* count the total number of _____ in the bonds and the non-bonding electrons

electrons

*the steps in drawing a Lewis structure:* make sure that the number of electrons used in the structure matches the number of _____ counted in step one. if the numbers don't match the structure probably as _____

electrons; double or triple bonds,

*the steps in drawing a Lewis structure:* although there are a few exceptions, most molecules and polyatomic ions have an _____ number of valence electrons

even

there are situations where more electrons are available than needed. this often results in _____ associated with the _____ in the Lewis structure

expanded octets, central atom

a ficticios charge that ignores electronegativity

formal charge

the charge than an atom would have if all bonding electrons are shared equally between the bonded atoms

formal charge

lower _____/lower _____ is the central atom

group number, electronegativity

dimers are _____ reactive than free radicals

less

if you get the same numbers for formal charge in both structures, the more negative charge is assigned to the _____

more electronegative element

with carbon, there is sometimes _____ central atom(s)

more than one

when formal charge cannot be avoided, _____ should reside on the most electronegative atom

negative formal charge

for metals in Groups 1, 2, and 3, the Lewis structure for the stable octet has _____ dots

no

many polyatomic ions contain _____ atoms contently bonded to each other

nonmetal

what does the element's symbol in a Lewis structure represent?

nucleus and core electrons

the element's symbol represents the _____ and _____ and dots arranged around the symbol represent the _____

nucleus, core electrons, valence electrons

1/2 number of bonding electrons =

number of bonds

formal charge =

number of valence electrons - (number of nonbonding electrons + 1/2 number of bonding electrons)

some compounds have formulas in which the total number of valence electrons is an odd number. in such cases, it is impossible to construct a Lewis structure with an _____ around each atom

octet ex: NO₂

*the steps in drawing a Lewis structure:* assume that all of the atoms in the molecule obey the _____ (except _____)

octet rule, hydrogen

halogen can only bond in _____ direction (no _____ bonds)

one, double or triple

*the steps in drawing a Lewis structure:* F atoms always form _____ bond(s), as do other _____ most of the time. exceptions occur when these _____ are central atoms bonded to other smaller _____ or to _____

one, halogens, halogens, halogen, oxygen

*the steps in drawing a Lewis structure:* all bonding electrons are _____

paired

an unpaired electron makes the molecules very _____

reactive

several possible correct structures

resonance structures

*the steps in drawing a Lewis structure:* avoid creating _____ or _____ when drawing Lewis structures

rings, cyclic structures

*Lewis structure* XeF₄

see 3.2 study guide

*the structure* NCl₃

see 3.2 study guide

*Lewis structure* SO₄²⁻

see 3.2 study guide, "Lewis Structures for Polyatomic Ions"

*Lewis structure* CO₃²⁻

see 3.2 study guide, "Resonance"

*draw the Lewis structure for the following polyatomic ions:* CrO₄²⁻

see 3.2 study guide, #11

*draw the Lewis structure for the following polyatomic ions:* C₂H₃O₂¹⁻

see 3.2 study guide, #11

*draw the Lewis structure for the following polyatomic ions:* NH₄¹⁺

see 3.2 study guide, #11

*draw the Lewis structure for the following polyatomic ions:* SO₄²⁻

see 3.2 study guide, #11

*draw Lewis structures for the following molecules or ions* ICl₄¹⁻

see 3.2 study guide, #12

*draw Lewis structures for the following molecules or ions* SeBr₄

see 3.2 study guide, #12

*draw Lewis structures for the following* CS₂

see 3.2 study guide, #13

*draw Lewis structures for the following* HCN

see 3.2 study guide, #13

*draw Lewis structures for the following* O₃

see 3.2 study guide, #13

write the Lewis structures for the main group elements in period 4

see 3.2 study guide, #4

*write the Lewis structure for the following atom and its ion* Al

see 3.2 study guide, #5

*write the Lewis structure for the following atom and its ion* Ba

see 3.2 study guide, #5

*write the Lewis structure for the following atom and its ion* Bi

see 3.2 study guide, #5

*write the Lewis structure for the following atom and its ion* I

see 3.2 study guide, #5

*draw the Lewis structure for each of the following molecule:* CCl₄

see 3.2 study guide, #8

*draw the Lewis structure for each of the following molecule:* H₂O

see 3.2 study guide, #8

*draw the Lewis structure for each of the following molecule:* NH₃

see 3.2 study guide, #8

*draw the Lewis structure for each of the following molecule:* OF₂

see 3.2 study guide, #8

*draw the Lewis structure of a molecule for:* bromine trifluoride, BrF₃

see 3.2 study guide, #9

*draw the Lewis structure of a molecule for:* carbon monoxide, CO

see 3.2 study guide, #9

*draw the Lewis structure of a molecule for:* formaldehyde, CH₂O

see 3.2 study guide, #9

*draw the Lewis structure of a molecule for:* hydrogen cyanide, HCN

see 3.2 study guide, #9

one pair of valence electrons shared between two atoms is a _____

single covaent bond

*the steps in drawing a Lewis structure:* construct the skeleton of te molecule using lines to indicate _____ between atoms using the guidelines which apply to most molecules and polyatomic ions

single covalent bonds

formal charge is used to distinguish between competing _____ or competing _____

skeletal structures, resonance structures

the elements symbol is placed in _____ with the _____ written to the right as a superscript outside the _____

square brackets, cation's charge, brackets

some of all formal charges in a polyatomic ion = _____

the charge of the ion

the magnitude of the positive charge is equal to _____

the number of dots (electrons) that were lost by the neutral atom

the magnitude of the negative charge is equal to _____

the number of electrons gained by the neutral atom in order to have four pairs

for metals, the number of dots represents _____

the number of electrons lost when forming a cation

what do the number of dots in main group metals tell us about the charges of the ions formed by these metals?

the number of electrons that the metal loses when it form an ion, so the magnitude of the positive charge

each element in a group has _____ number of _____ so the Lewis structure for each element in the same group has ____ number of _____

the same, valence electrons, the same, dots

the single line between the atoms represents _____ in a _____ bond. the other pairs of electrons are _____

the shared pair of electrons, covalent, non-bonding or lone pairs

*the steps in drawing a Lewis structure:* nitrogen atoms form _____ bonds and carbon atoms form _____ bonds

three, four

*the steps in drawing a Lewis structure:* oxygen atoms normally form _____ bonds and don't often bond to _____ in compounds with other elements (except _____--_____)

two, each other, hydrogen peroxide, H₂O₂

*the steps in drawing a Lewis structure:* a Lewis structure must account for all of the _____ in the atoms that make up the molecule

valence electrons

*the steps in drawing a Lewis structure:* determine the total number of _____ in the molecule

valence electrons

what do the dots in a Lewis structure represent?

valence electrons

*the steps in drawing a Lewis structure:* arrange the remaining _____ to satisfy the _____ for all atoms

valence electrons, octet rule

*the steps in drawing a Lewis structure:* subtract the number of _____ used for bonding in the skeleton. each lines represents _____

valence electrons, two electrons

*how many total valence electrons are present in the following molecules?* CCl₄

32

for any main group (representative) element we can write Lewis structures using the following steps:

-write the element symbol -determine the number of valence electrons -imagine a square around the element symbol. place one dot for each valence electron on each side of the square, starting with the side of the square to the right of the element's symbol. proceed clockwise around the square. -if more than four valence electrons are present, begin pairing the dots only after all four sides of the square have one dot (remember Hund's rule: electrons get their own orbital and only pair up when each orbital in a sublevel has one electron).

some of all formal charges = _____ in a neutral molecule

0

*how many electrons would be needed by the following nonmetals elements to acquire a stable octet?* F

1

*how many electrons would be needed by the following nonmetals elements to acquire a stable octet?* O

2

*how many electrons would be needed by the following nonmetals elements to acquire a stable octet?* N

3

which molecule would you expect to be a free radical? CO CO₂ N₂O NO

NO

*three possible Lewis structures are shown for the thiocyanate ion below.* *a. determine the formal charges in each structure* *b. based on the formal charges, which Lewis structure is the dominant one?* [N/C///S]¹⁻ [N//C//S]¹⁻ [N///C/S]¹⁻

[N//C//S]¹⁻

the sulfate ion (SO₄²⁻) has two resonance structures shown below. based on the formal charges, which one is the most dominant? a. four single bonds b. two single bonds, two double bonds

b


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